It is of utmost importance and an absolute requirement that the lasers and their corresponding beams are correctly focused so as to register with and to intersect the isocenter which corresponds to the point at which the high energy beams from the radiation head intersect the epicenter axis of rotation. It is the correct and accurate focusing of the laser beams, or light beams, upon said isocenter which will assure that throughout rotation of the accelerator high energy head throughout 360.degree. about the aforesaid epicenter axis, that the high energy rays will impinge upon the isolated area of the patient to be treated when said isolated area is likewise located at said isocenter.
Accordingly, when the patient in a horizontal position rests upon the universally adjustable platform of the patient couch adjacent and spaced from the teletherapy head, and the isolated area to be treated has impinged thereon the respective rays of at least three of the four laser beams, it is then assured that the high energy beam from said head will impinge upon only said isolated area regardless of its position of rotation with respect to the epicenter axis.
It is important, therefore, that the laser beams be focused and that they are, at all times, directed to and intersect said isocenter. Since these lasers are adjustable, it is often possible that they may be accidentally moved out of adjustment and thus, would give an incorrect registry and their beams would no longer be in focus with the above-described isocenter. The problem, therefore, is to have assurance that the lasers or other light sources are so adjusted for use in conjunction with the linear accelerator that the beams are correctly and accurately focused upon said isocenter.